Engineers are constantly seeking ways to reduce undesirable engine emissions. One strategy is to seek ways to improve performance of fuel injection systems. Over the years, engineers have come to learn that engine emissions can be a significant function of injection timing, the number of injections, injection quantities and rate shapes. However, it has been observed that an injection strategy at one engine operating condition may decrease emissions at that particular operating condition, but actually produce an excessive amount of undesirable emissions at a different operating condition. Thus, a fuel injection system with a variety of capabilities to produce a variety of injection strategies can better perform and reduce emissions at all engine operating conditions than a fuel injection system limited in its control over injection timing, number, quantity and rate shapes. Further, increases in the ability to vary injection rates, injection numbers, injection quantities and rate shapes can lead to more research on, and discovery of, improved injection strategies at different operating conditions.
One apparent attempt to provide a fuel injection system that can quickly vary the pressure of injections is disclosed in “Heavy Duty Diesel Engines—The Potential of Injection Rate Shaping for Optimizing Emissions and Fuel Consumption”, presented by Messers, Bemd Mahr, Manfred Durnholz, WilhelmPolach, and Hermann Grieshaber, Robert Bosch GmbH, Stuttgart, Germany, at the 21st International Engine Symposium, May 4–5. 2000, Vienna, Austria. This reference teaches a common rail system and a directly controlled fuel injector that purportedly has the ability to inject medium pressure fuel directly from the rail, or utilize the fuel common rail to pressure intensify fuel within the injectors for injection at relatively high pressures. The magnitude of high pressure of pressure intensified injection will be, in part, a function of the pressure of the fuel acting on a pressure intensifier within the fuel injector.
While this fuel injection system theoretically may have the ability to produce multiple injections, each at different pressures and close in time, the fuel injection system does have drawbacks. For instance, the fuel used to actuate the pressure intensified injection and the fuel being injected directly from the common rail have the same source, i.e., the common rail. Thus, they are both at common rail pressure. In situations in which there is insufficient time to alter the pressure within the common rail between injections, the high pressure of the pressure intensified injection is dependent on the medium pressure injection of the common rail injection, or vice versa. For instance, the pressure of a main injection that is pressure intensified is limited by the pressure of a pilot injection directly injected from the common rail. Thus, although the pressure of the high pressure injection is greater than the pressure of the medium pressure injection, the Bosch fuel injection system lacks the capability to vary the pressure of the high pressure injection without also varying the pressure of the medium pressure injection.
The present invention is directed to increasing the capabilities of fuel injection systems.